In a typical radio communication device, a transmitter is coupled to an antenna to provide a transmission path for communication signals. The transmitter includes a power amplifier to amplify the signals before transmission. The behavior of a transmitter may be affected by its operating environment. For example, a transmitter operating near an electromagnetically reflective structure may be susceptible to energy reflected back through the antenna into the transmitter. Reflective energy may be detrimental to transmitter performance, particularly to the performance of the power amplifier. The power amplifier design often relies upon a constant load impedance in maximizing gain, efficiency, power output level, and other like parameters. To protect against changes in load impedance as a result of reflected energy, an isolator is often inserted between the antenna and the power amplifier of a transmitter in order to protect the power amplifier from reflected energy. The isolator directs the reflected energy to an absorptive load termination. Although the isolator generally works well, the isolator adds significant cost, size, and weight to the design of a radio communication device.
Another prior art solution to the problem of reflected energy incorporates a directional coupler to detect the reflected energy and adjusting the gain of the power amplifier accordingly. Generally, to minimize the potential of damage to the power amplifier, the gain to the power amplifier is reduced when high levels of reflected energy are present. Directional couplers and associated circuitry also add cost and complexity to a radio design.
FIG. 1 is a block diagram showing a prior art linear transmitter 100. Linear transmitters, which are typically used in quadrature amplitude modulation (QAM), must provide a predictable response in order to reliably transmit complex base band signals. In the linear transmitter 100, a signal source 110 provides a complex baseband signal to transmitter circuitry 120. Amplification circuitry 122 amplifies the signal for transmission through an antenna 130. As is typical in the prior art, a feedback signal from a negative feedback correction loop 124 is combined with the source signal in a summer 121. The feedback loop is used to improve the linearity of the power amplifier. In so doing, the level of energy transmitted on adjacent channel frequencies, known as splatter, is reduced. Such adjacent channel energy may be disruptive to negative feedback correction. Further included in the prior art transmitter 100 is an isolator 126 situated between the antenna 130 and the remaining transmitter circuitry.
The isolator presents a constant load impedance to the power amplifier irrespective of the impedance presented to the isolator by the antenna. The use of the isolator thereby avoids unpredictable variations in power amplifier gain and phase characteristics which would occur if the antenna were connected directly to the power amplifier. Those skilled in the art will understand that a varying load impedance will cause the power amplifier gain to change, thus altering the effectiveness of the feedback correction loop. Such load variations may also result in phase changes within the power amplifier which may make the feedback loop unstable. Unstable operation may result in severe interference to other communication services and/or the destruction of the linear transmitter. Thus, the isolator protects the power amplifier from load impedance changes and associated reflected energy from the antenna 130 during the transmission.
It is desirable to provide a linear transmitter while avoiding the cost, size, and weight issues associated with isolators. Such linear transmitters must be capable of rapidly adjusting to changes in operating environment to maintain a stable, linear response.